The present invention refers to a T-shaped circulator with low symmetry based on a two-dimensional photonic crystal with square lattice.
Photonic crystals are periodic structures built with materials that have different refractive indices. In such structures, there is a forbidden range of frequencies, known as photonic band gap. Electromagnetic waves with frequency located within this range are reflected by the crystal. The occurrence of this phenomenon is exploited for the design of most devices that are based on photonic crystals.
Nonreciprocal components such as circulators and isolators are used in communication systems to reduce undesirable reflections that cause instability in generators and amplifiers, as well as loss of performance in these systems.
Because of the nonreciprocity, circulators carry out the transmission of electromagnetic signals in one direction (clockwise or counterclockwise) determined by the sign (positive or negative) of an applied DC magnetic field. Thus, in a 3-port circulator, for example, there may be the following modes of operation (input port→output port): 1→2, 2→3 and 3→1 (clockwise); or 1→3, 3→2 e 2→1 (counterclockwise). In an ideal circulator, the power of the signal present at the input port is entirely transferred to the output port (without loss).
As a result of researches related to the development of new devices based on photonic crystals, some patents related to circulators based on these structures have been deposited.
For example, the circulator referred in the patent US20120243844 is built on a two-dimensional photonic crystal with triangular lattice, consisting of holes filled with air and etched in a dielectric material. Its resonant cavity is comprised of a magneto-optical rod and of holes with changed diameters (when compared to other holes of the photonic crystal). The device in question has three ports.
The report describing the patent US20120251048 refers to a circulator with four ports, based on the coupling of two resonant cavities. Both cavities are formed by a magneto-optical rod and by holes with modified diameters.
On the other hand, the patent US20130223805 relates to a circulator built on a two-dimensional photonic crystal with square lattice of holes filled with air, etched in a dielectric material. The resonant cavity of this circulator is characterized by having four magneto-optical rods surrounding a central dielectric rod and three dielectric rods with modified diameters.
All these devices are used mainly to perform the isolation function, that is, the protection of signal sources against parasitic reflections coming from not matched loads connected to a communication system.
In a circulator with three ports, for example, assuming that port 1 is connected to a signal source (input), port 2 is connected to the circuit that will receive the signal from the signal source (output), and port 3 is connected to an ideally matched load, the circulator will operate as following.
Considering the clockwise propagation (1→2, 2→3 and 3→1), the signal coming from input (port 1) is sent to output (port 2). However, parasitic reflections that might be originated at the output (port 2) do not return to input (port 1). They will be directed to port 3, in which is connected an ideally matched load that absorb them, protecting the signal source connected to port 1.